Small surface-mounted hardware protection devices (surface-mounted devices SMD) or fuses are required for many circuit applications, e.g. in automotive engineering, measurement and control technology etc. For technical and cost reasons, such fuses are realised as conventionally used in printed circuit technology. SMD fuses, which comprise melting fuses, are mostly positioned and placed automatically by automatic pick-and-place machines on FR4 printed circuit boards. SMD fuses are subsequently soldered by means of reflow soldering processes or wave soldering processes onto the printed circuit board. FR4 printed circuit board materials or Al2O3 ceramics are used for example as base materials for SMD fuses, i.e. all conventional base materials for the production of printed circuit boards.
Fuses comprise a fuse element arranged on a base support, which fuse element comprises copper for example. The fuse element is usually used for protection from overcurrents and thus protects the subsequent electronic components.
Fuses come with the disadvantage that the base supports usually have limited operating temperatures. The operating temperature of a base support made of FR4 base material is thus only 200° C. for example. Higher temperatures damage the FR4 base material. In this case, the material delaminates and the fuse element which mostly consists of a cover film detaches from the base support. Decomposition and charring of the material occurs after a short period of time. Conductive layers are produced by the charring, with a comparatively low electrical resistance, which then produce impermissibly low insulation resistances.
In order to remedy this problem it is known to produce the base support from an Al2O3 ceramic material, which can withstand substantially higher temperatures than 200° C. for example without being damaged. It has proven to be disadvantageous however that the coefficient of thermal expansion (CTE) of said Al2O3 ceramic material is mostly less than 8 ppm/K and thus differs strongly from the coefficient of thermal expansion CTE of copper, which is 17 ppm/K. As a result of this high difference between the coefficient of thermal expansion of the base support made of Al2O3 ceramic material and copper, mechanical tensions occur between the copper fuse element and the ceramic base support. This leads to an increased likelihood of breakage. Furthermore, ceramic substrates are generally very brittle and drain a large amount of thermal energy from the fuse element. As a result, fuses with low nominal currents and rapid characteristics on this Al2O3 ceramic material are difficult to realise. Furthermore, these ceramic fuses frequently break once the fuse element is loaded by torsion or bending.
There is a disadvantage in the prior art that thermal fuses cannot be soldered on the basis of SMD by means of a reflow soldering process for example. The reason is that the known thermal fuses will trigger immediately under the thus occurring high temperatures in a range of 240° C. to 265° C.